slub: fix data loss and overflow in krealloc()

Commit 2cd8231796b5 ("mm/slub: allow to set node and align in
k[v]realloc") introduced the ability to force a reallocation if the
original object does not satisfy new alignment or NUMA node, even when
the object is being shrunk.

This introduced two bugs in the reallocation fallback path:

1. Data loss during NUMA migration: The jump to 'alloc_new' happens
before 'ks' and 'orig_size' are initialized. As a result, the
memcpy() in the 'alloc_new' block would copy 0 bytes into the new
allocation.

2. Buffer overflow during shrinking: When shrinking an object while
forcing a new alignment, 'new_size' is smaller than the old size.
However, the memcpy() used the old size ('orig_size ?: ks'), leading
to an out-of-bounds write.

The same overflow bug exists in the kvrealloc() fallback path, where the
old bucket size ksize(p) is copied into the new buffer without being
bounded by the new size.

A simple reproducer:

// e.g. add to lkdtm as KREALLOC_SHRINK_OVERFLOW
while (1) {
void *p = kmalloc(128, GFP_KERNEL);
p = krealloc_node_align(p, 64, 256, GFP_KERNEL, NUMA_NO_NODE);
kfree(p);
}

demonstrates the issue:

==================================================================
BUG: KFENCE: out-of-bounds write in memcpy_orig+0x68/0x130

Out-of-bounds write at 0xffff8883ad757038 (120B right of kfence-#47):
memcpy_orig+0x68/0x130
krealloc_node_align_noprof+0x1c8/0x340
lkdtm_KREALLOC_SHRINK_OVERFLOW+0x8c/0xc0 [lkdtm]
lkdtm_do_action+0x3a/0x60 [lkdtm]
...

kfence-#47: 0xffff8883ad756fc0-0xffff8883ad756fff, size=64, cache=kmalloc-64

allocated by task 316 on cpu 7 at 97.680481s (0.021813s ago):
krealloc_node_align_noprof+0x19c/0x340
lkdtm_KREALLOC_SHRINK_OVERFLOW+0x8c/0xc0 [lkdtm]
lkdtm_do_action+0x3a/0x60 [lkdtm]
...
==================================================================

Fix it by moving the old size calculation to the top of __do_krealloc()
and bounding all copy lengths by the new allocation size.

Fixes: 2cd8231796b5 ("mm/slub: allow to set node and align in k[v]realloc")
Cc: stable@vger.kernel.org
Reported-by: https://sashiko.dev/#/patchset/20260415143735.2974230-1-elver%40google.com
Signed-off-by: Marco Elver <elver@google.com>
Link: https://patch.msgid.link/20260416132837.3787694-1-elver@google.com
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>